This blog provides updated forecasts and comments on current weather or other topics

Friday, November 27, 2009

Sun, Fog, and Rain Takes a Holiday

Yes, it is true. The weather stretching into the middle of next week is going to be very different from the rain and wind of November. We are talking DRY now. But with dry this time of year comes the potential for fog.

Take today. Yesterday's front and moderate rain has pushed through western Washington and sunny skies have spread over the lowlands (see satellite picture), including much of the WA coast. There are some low clouds on the western slopes of the Cascades and the coastal mountains--extending to the mountain crests. Temps will be in the upper 40s. Perfect raking and walking weather.

Tomorrow a weak disturbance will bring increasing clouds and rain for the northwest portion of the state, but the computer models suggest the lowlands will be mainly rainshadowed by the Olympics and coastal mountains. Want dry? Go south!

On Sunday the ridge builds up again and rain will end. In fact, for much of next week a very high amplitude ridge aloft will provide dry conditions. Little wind.

The only problem with all this is that when high pressure and clearing skies are parked over us, with weak low-level winds, there is a real chance for fog this time of the year.

Now, I can be pretty sure about this forecast...using a technology I often don't talk about, but use all the time--ensemble predictions. The National Weather Service runs a forecast ensemble--many forecasts all starting slightly differently. If the atmosphere is sensitive and the forecasts uncertain, the slight changes in the initial state will cause the predictions to vary greatly. If there are all on the same page...we can have more confidence in the predictions. Take a look at the ensemble predictions for Sunday (this is called a spaghetti diagram for obvious reasons). Virtually all the predictions are going for the ridging!Look at the spaghetti diagram for Tuesday afternoon....amazing agreement that far out. Ridging is in!And remember, we are now finishing the historically wettest period of the year. We have ascended the meteorological peak...we now move slowly (very slowly at first) towards summer. The worst may well be over...especially considering this is an El Nino year.

Fog...urgh! The valleys east of the cascades can be stuck in the gray for weeks under those conditions.

The only excitment I look for in the extended is sometimes those ridges build so strong to the north, a low south from Canada can slip or retrograde back off the coast. Rare but good snow situation for the inland NW. The euro shows that but models always over estimate that pattern

Ahhhh, the perfect day! Leftovers, a walk in the woods viewing the varied fungal bounty of the rain with a friend, sitting in the sun on coastal cliffs, fall cleanup in the garden in balmy stillness. Love the storms and love these rare, fair, fall days. Much to be thankful for. :)

The long range forecast, seen on weather.com and accuweather.com, is calling for much cooler conditions for western washington (highs around 40) and eastern washington (highs around 30) in the next couple weeks....I wonder if this will be true.

Thank you for providing the elegant ensemble diagrams. Looking forward to drying out; water in the basement wasn't very fun. Lots of yard work to do to prepare for spring next year, so here's my chance.

Cliff mentioned about low clouds and or fog hanging around the Western slopes of the Cascades today, we`ll his statement and satellite pic are VERY TRUE!

Was up at Mt.Si today with a group of Mountaineer hikers, and it was pretty foggy indeed up there, with a little snow as well, maybe like 3 or 4" worth with temp of 35. The fog gave the forest a very mystical appearance.

I appreciate your trying to cheer up the area residents with the statement “and remember, we are now finishing the historically wettest period of the year”, but in reality, I think that’s a little too rosy. While it is true that Sea-Tac’s climatological wettest day (1961-1990 average) is November 23 with .209 inches, if you look at a number of stations around Puget Sound, and smooth out some of the day-to-day jitters in the average precipitation that are just statistical noise, I think the maximum precipitation day is probably right around December 1.

Futhermore, if you express it in terms of “wettest period”, the time series of daily normal precipitation at most Puget Sound stations is very much skewed, or in other words, the wettest day is not the middle of the wettest period. At most stations, the period during which daily precip is at or above 85% of the climatological wettest day spans from around November 10 clear through to mid-January. So, by that definition, really we are only two and a half weeks into the wettest nine weeks of the year. Sorry to be the “wet blanket”.

There is indication in the medium-range models that cooler air from the Canadian interior may surge into the lowlands through the Frasier Valley late next weekend into the second weekend of December. It's still a bit early, but both the GFS and the European have shown indications (the following are from the Friday 00z runs):

Interesting, .. about the time valid stamped for the farthest out, reaching ensemble that you've pointed to Cliff - i.e. the 2nd, is about the same date that I've been speculating that the next main, broader wave of cold will begin expanding south. Main and broader cold, certainly appears to have been in a general retraction mode for the past week or so.